The IPv6 switch: when to turn it on and what else to change

The General page has an IPv6 switch, off by default. While it is off the core refuses IPv6 connection requests and programs normally fall back to IPv4, which most people never notice.

When you need it on

• Your campus or home connection only hands out an IPv6 address and pure IPv4 cannot get out;
• You need sites that publish only AAAA records and no A record;
• The node's own entry point is an IPv6 address.

The switch alone is not enough

DNS also has to be allowed to return AAAA records, otherwise resolution breaks before anything connects:

dns:
  enable: true
  ipv6: true

Rules need attention too: IP-CIDR only matches IPv4, so IPv6 ranges must be written as IP-CIDR6. Most rule sets copied from elsewhere contain no IPv6 entries at all, so once you enable it that traffic falls straight through to MATCH. GEOIP still works for IPv6. Under TUN mode, confirm the virtual adapter actually received an IPv6 address — run ipconfig and look at the Clash adapter block.

When enabling it makes things slower

This is common where the ISP's IPv6 quality is poor: the browser tries IPv6 first, waits several seconds for a timeout and only then falls back to IPv4, so every new site hangs briefly before loading. Test by turning the switch off and revisiting the same site — if it is instantly faster, that was the cause. Either leave it off, or set ipv6: false under DNS only and keep system-level IPv6 available.